Which problem could be caused mixed "different" source despite are relative at the same branch (in that case the last release 5.0.1)?

To be honest I don't know, but, in the above mentioned announcement it is stated that:

pkgsrc-2009Q2 branch is now deprecated

and, pkg_install infrastructure has been updated.

Later means that pkg_add, pkg_info, pkg_delete, pkg_admin ... etc have been "changed", and that could affect your existing system.

I would gently suggest you update to 2009Q3 if possible.

Take care.

__________________
The best way to learn UNIX is to play with it, and the harder you play, the more you learn.
If you play hard enough, you'll break something for sure, and having to fix a badly broken system is arguably the fastest way of all to learn. -Michael Lucas, AbsoluteBSD

__________________
The best way to learn UNIX is to play with it, and the harder you play, the more you learn.
If you play hard enough, you'll break something for sure, and having to fix a badly broken system is arguably the fastest way of all to learn. -Michael Lucas, AbsoluteBSD